1. Field of the Invention
The present invention relates to an optical condition design method for a compound-eye imaging device.
2. Description of the Related Art
A compound-eye imaging device is known which comprises an optical lens array having multiple micro optical lenses, and a solid-state imaging element (photodetector array) for imaging multiple unit images of a target object which are formed by the respective optical lenses of the optical lens array so as to reconstruct the multiple unit images into a high definition reconstructed image (refer to e.g. Japanese Laid-open Patent Publication 2001-61109). FIG. 8 is a schematic perspective view of such a conventional compound-eye imaging device 100.
As shown in FIG. 8, the compound-eye imaging device 100 has an optical lens array 101 having micro optical lenses L11, L12 . . . L33 equidistantly arranged in a matrix of rows and columns, and a solid-state imaging element 102 for imaging unit images K11, K12 . . . K33 based on images formed by the respective optical lenses L11, L12 . . . L33. The solid-state imaging element 102 has a major surface having many pixels g which are grouped into pixel groups G11, G12 . . . G33 in areas of the solid-state imaging element 102 corresponding to the respective optical lenses L11, L12 . . . L33. The pixel groups G11, G12 . . . G33 image or capture the corresponding unit images K11, K12 . . . K33, respectively, and convert the unit images K1, K12 . . . K33 to electric signals as image information, and further send the electric signals converted from the respective unit images K11, K12 . . . K33 to an image processor 103 which forms a reconstructed image from the electric signals of the unit images K11, K12 . . . K33.
Here, a mechanism to allow a compound-eye imaging device to form, from multiple unit images, a reconstructed image having a higher definition than that of each unit image will be explained with reference to FIG. 8. In the compound-eye imaging device 100, the optical lenses L11, L12 . . . L33 are distributed on the optical lens array 101 in a matrix of rows and columns, so that the respective unit images K11, K12 . . . K33 formed on the solid-state image element 102 are images which are viewed at angles slightly different from one another relative to a target object positioned in front of the compound-eye imaging device 100 (i.e. images with parallax). By using such unit images, as image information, which are thus slightly different from one another to have slightly different information, it is possible to form a reconstructed image based on the larger amount of information, making it possible for the reconstructed image to have a higher definition than that of each unit image. In other words, in order to obtain larger amount of information of unit images, it is desirable that the respective unit images do not have portions of the same image information.
However, when considering specific pixels g in the unit images K11, K12 . . . K33, different pixels g may in some cases have the same image information. More specifically, light emitted from the same portion on a target object may in some cases pass through different paths so as to be imaged by two different pixels g that are, for example, a pixel g in the pixel group G11 and a pixel g in the pixel group G12. This causes the two pixels g to have the same image information based on an image of the same portion on the target object. In other words, although the unit images K11 and K12 are those obtained by imaging or capturing the target object at different angles, the unit images K11 and K12 may in some cases have partially the same image information. In such cases, the difference of image information between the unit images K11 and K12 is reduced thereby so that the definition of the formed reconstructed image is reduced.
The case where different unit images have partially the same image information in the compound-eye imaging device 100 will be described in more detail with reference to FIGS. 9A and 9B. FIG. 9A is a schematic side view of the compound-eye imaging device 100, placed vertically, in a mode of collecting lights from a target object T by the optical lenses L11, L12 . . . L33, in which the target object T is positioned in front of the compound-eye imaging device 100. FIG. 9B is a schematic enlarged side view of a portion of FIG. 9A as indicated by the dashed circle 110. In FIG. 9B, the three optical lenses L11, L21 and L31 in the leftmost column in the compound-eye imaging device of FIG. 8 together with the three pixel groups G11, G21 and G31 are representatively shown, in which the solid-state imaging element 102 is assumed to have 8 (eight) pixels g1 to g8 for each of the optical lenses L11, L21 and L31.
Both FIG. 9A and FIG. 9B show paths of lights collected by the respective optical lenses L11, L21 and L31 to reach the respective pixels of the solid-state imaging element 102, in which such light paths for the optical lenses L11, L21 and L31 are shown by solid lines, coarse dashed lines and fine dashed lines in order from top to bottom. More specifically, in the case of the optical lens L11, for example, the uppermost light denoted by h1 passes through the optical lens L11, and is collected onto the lowermost pixel denoted by g8 in the pixel group G11. Similarly, the n-th light from the top which can be denoted by hn passes through the optical lens L11, and is collected onto the (9−n)th pixel from the top which can be denoted by g(9−n). The lowermost light denoted by h8 passes through the optical lens L11, and is collected onto the uppermost pixel denoted by g1.
Now, assuming that the target object T is positioned in front of, and at a predetermined distance, from the compound-eye imaging device 100, FIGS. 9A and 9B show the case where light from a point p1 on the target object T is collected by the optical lens L11 as light h2 and imaged by pixel g7 in the pixel group G11, and is at the same time collected by the optical lens L31 as light h1 and imaged by pixel g8 in the pixel group G31. Thus, the image information of the pixel g7 in the unit image K11 is the same as the image information of the pixel g8 in the unit image K31, both based on the point P1 on the target object T. This indicates that points or portions on the target object T where the paths of lights h1, h2, h3 . . . intersect, that are p1, p2, p3 . . . , are imaged as the same image information in the different pixel groups G11, G12 . . . G33. Thus, it is understood that in the conventional compound-eye imaging device 100 with the regularly arranged optical lenses L11, L12 . . . L33, the intersection points between lights h1, h2, h3 . . . are present and concentrated on a specific plane, because the paths of lights h1, h2, h3 . . . are parallel to each other. Accordingly, if the target object T is located closer to the specific plane, it causes the respective unit images K11, K12 . . . K33 to have more portions of the same image information. This reduces the differences of image information between the unit images K11, K12 . . . K33, thereby reducing the definition of the reconstructed image.
Based on the knowledge described above, the present inventors conceived randomly arranging the respective optical lenses L11, L12 . . . L33 to prevent the intersection points between the paths of lights h1, h2 . . . h8 to the respective optical lenses L11, L12 . . . L33 from concentrating on a specific plane. This can be more specifically described with reference to FIG. 10 which is a schematic plan view of the optical lens array 101 of the conventional compound-eye imaging device 100. As shown in FIG. 10, the random arrangement is done by arranging the respective optical lenses L11, L12 . . . L33 at positions which are offset from the normal positions regularly arranged in a matrix of rows and columns. This makes it possible to prevent the paths of lights h1, h2 . . . h8 to the respective optical lenses L11, L12 . . . L33 from being parallel to each other as shown in FIG. 11, which is a schematic view showing how lights h from the target object T are collected in the conventional compound-eye imaging device 100 with the randomly arranged optical lenses L11, L12 . . . L33, thereby preventing occurrence of a plane on which light intersection points are concentrated.
This makes it possible to reduce portions of the same image information of the respective unit images K11, K12 . . . K33, thereby increasing the definition of the reconstructed image. However, depending on a random number used to determine the arrangement of the optical lenses, the random arrangement of optical lenses described above may incidentally cause the intersection points between the paths of lights h1, h2 . . . h8 to the respective optical lenses L11, L12 . . . L33 to be incidentally concentrated on a specific plane. In this case, it is not possible to increase the definition of the reconstructed image. In short, the random arrangement of optical lenses does not make it possible to always stably increase the definition of the reconstructed image.